1. Field of the Invention
The present invention relates to an ultra-narrow band fluorine laser apparatus for supplying laser beams from a fluorine laser as an exposure light source of an exposure apparatus.
2. Description of the Related Art
There are various requirements for the performance of a lithographic exposure apparatus including resolution, alignment accuracy, processing capability and reliability of the apparatus. Resolution R which directly relates to fineness of a pattern is expressed by R=k·λ/NA where k represents a constant; λ represents an exposure wavelength; and NA represents the numerical aperture of a projecting lens. Therefore, the shorter the exposure wavelength λ is, the more preferable resolution becomes. In exposure tools according to the related art, therefore, an i-line (having a wavelength of 365 nm) from a mercury lamp or a krypton fluorine (KrF) excimer laser having a wavelength of 248 nm is used as an exposure tool light source. The machines are therefore referred to “i-line exposure tools” or “KrF exposure tools”.
Exposure tools utilizing an argon fluorine (ArF) excimer laser having a wavelength of 193 nm as an exposure light source have been put in use as next generation exposure tools for fine processing. Those machines are referred to as “ArF exposure tools”. An ArF exposure tool uses a narrow band ArF excimer laser in a bandwidth as small as about 0.6 pm and an achromatic lens made of two types of materials is used as a reducing projection optical system.
Further, referring to next generation lithographic exposure tools to replace the above-described ArF exposure tools, studies are in progress on fluorine exposure tools utilizing a fluorine laser having a wavelength of about 157 nm as a light source.
A fluorine laser has two oscillation beams (also referred to as “oscillation lines”) with different wavelengths and optical intensities. It is said that those beams have wavelengths of 157.5233 nm and 157.6299 nm respectively and that each of the oscillation beams has a bandwidth of about 1 pm.
When such a fluorine laser is used for exposure, in general, it is considered advantageous to use only one line with a wavelength (157.6299 nm) having high intensity (hereinafter referred to as “single line configuration”), and one or two prisms have been used for such a single line configuration according to the related art.
However, since the line has a bandwidth of about 1 pm, it is considered necessary to use a catadioptric reducing projection optical system which is said to be usable in a bandwidth ten times wider than that of totally refractive optical system utilizing only lenses, as a reducing projection optical system of an exposure tool.
Operating characteristics and the like of fluorine lasers are described, for example, in “The Review of Laser Engineering, Vol. 19, No. 11, pp. 2-24” (reference 1).
Results of experiments on the one line selection are reported in, for example, “SPIE, 24th International Symposium on Microlithography, February 1999” (reference 2).
A catadioptric type system as described above necessitates a new design because it is different from refractive reducing projection optical systems formed by only lenses commonly used in conventional exposure tools (i.e., ArF exposure tools), which has resulted more problems in this type of system than in a totally refractive reducing projection optical system.
For the reasons described below, it has been difficult to provide a laser oscillation in a bandwidth as narrow as about 0.2 to 0.3 pm which is regarded usable in a totally refractive reducing projection optical systems which are more advantageous than the catadioptric type.
While a partial reflection coating must be used in an etalon or the like whose band can be narrower than that in the case where a prism is used, such a partial reflection coating may not provide sufficient light-resisting strength.
That is, in a laser with a short wavelength like a fluorine laser (i.e., a laser having a high photon energy), since a great amount of laser light is absorbed by many optical materials, even a small amount of impurity can cause damage of a partial reflection coating provided on an etalon or the like attributable to a temperature rise beyond the melting point thereof because of the absorption of laser light by the optical materials forming the coatings.
Further, while a substrate having a high surface accuracy is required for an etalon, for example, a substrate (optical material) usable in a wavelength λ=157 nm of a fluorine laser and whose base material is calcium fluoride results in a problem in that it is difficult to achieve surface accuracy of λ/100 which is normally required for an etalon.
It is an object of the invention to provide an ultra-narrow band fluorine laser apparatus having a line width as narrow as 0.2 to 0.3 pm without using an element such as an etalon for achieving a narrow band.